Update subtype from 'imagedata' to 'datacube'

[emmanuelmathot]

This fixes #57

[vincentsarago]

@emmanuelmathot I don't think it's as simple as just renaming the type

Right now we have datacube being a RasterStack https://github.com/sentinel-hub/titiler-openeo/blob/main/titiler/openeo/processes/implementations/data_model.py#L11. ImageData is one element of the datacube.

if we take the apply_pixel_selection process, it would not work on a ImageData but only Stack, I'm not quite sure what to do.

It seems that we could play with the dimensions https://github.com/Open-EO/openeo-processes/blob/6141771129d58d2a292db3b91d80c812c49f4e52/ndvi.json#L11-L30, maybe an ImageData is a bands of a data cube 🤷 (again I feel openeo as assets as band baked in, which is not always the case)

👀 https://github.com/Open-EO/openeo-processes/blob/master/meta/implementation.md#data-cubes